Abstract:

The present invention is a method for seaming a polymeric material. The
method involves forming an interface between a plurality of separate
polymeric sheets of materials. Next, heat and pressure are applied to
facilitate the diffusion of the polymer molecules at the interface. A
diffusion weld is made when the polymer molecules diffuse across the
interface.

2. The composite thermoset plastic sheet of claim 1 where the seam does
not include a separate adhesive.

3. The composite thermoset plastic sheet of claim 1 wherein at least one
of the component thermoset plastic sheets comprises a polyimide.

4. The composite thermoset plastic sheet of claim 1 wherein at least one
of the component thermoset plastic sheets includes a coating.

5. The composite thermoset plastic sheet of claim 1 wherein the plurality
of component thermoset plastic sheets further comprises at least a first
component thermoset plastic sheet and a second component thermoset
plastic sheet, wherein the first component thermoset plastic sheet is of
a different material than the second component thermoset plastic sheet.

6. The composite thermoset plastic sheet of claim 1 wherein at least one
component thermoset plastic sheet includes a coating, and the coating
comprises polytetrafluoroethylene (PTFE).

7. The composite thermoset plastic sheet of claim 1 wherein the component
thermoset plastic sheets are comprised of a polymer selected from the
group consisting of polyimides, polybenzoxazoles, polyimide filled
polytetrafluoroethylene, and any combination thereof.

8. A composite plastic sheet comprising:a plurality of component plastic
sheets;a seam, wherein at least two component plastic sheets are in
direct contact in the seam; anda diffusion weld connecting at least two
component plastic sheets in the seam where at least one of the connected
component plastic sheets is a thermoset plastic.

9. The composite plastic sheet of claim 8 wherein at least one component
plastic sheet includes a coating.

10. The composite plastic sheet of claim 8 wherein the component plastic
sheets include at least two different types of plastic sheets.

11. A composite plastic sheet comprising:a plurality of polymeric sheets
including at least a first polymeric sheet and a second polymeric sheet;a
seam, wherein at least the first polymeric sheet and the second polymeric
sheet are in direct contact in the seam; anda plurality of spot diffusion
welds connecting the plurality of polymeric sheets in the seam.

12. The composite plastic sheet of claim 11 where at least one polymeric
sheet is thermoset.

13. The composite plastic sheet of claim 11 where each spot diffusion weld
is isolated from the other spot diffusion welds.

14. A method of seaming polymeric sheets comprising:(a) providing a
plurality of polymeric sheets, wherein at least one polymeric sheet is a
thermoset polymeric sheet;(b) overlapping the polymeric sheets to produce
an interface such that the polymeric sheets are in direct contact at the
interface; and(c) applying heat and pressure to the interface to
diffusion weld the polymeric sheets.

15. The method of claim 14 wherein at least one polymeric sheet includes a
coating and acore.

16. The method of claim 15 wherein the coating is a metallic material, the
method further comprising:removing the coating from a portion of the core
such that there is no coating between the polymeric sheets at the
interface before step (b).

17. The method of claim 15 wherein step (b) further comprises positioning
the polymeric sheets such that the coating directly contacts another
polymeric sheet at the interface.

18. The method of claim 15 wherein the core comprises a thermoset
material.

20. The method of claim 14 further comprising tensioning the polymeric
sheets before step (c).

21. The method of claim 14 wherein the plurality of polymeric sheets
includes at least two polymeric sheets comprised of different materials.

22. The method of claim 14 wherein the plurality of polymeric sheets are
thermoset plastics.

23. A method for seaming a polymeric material, comprising:(a) providing at
least two polymeric sheets including at least one edge, a first side and
a second side, where the first side comprises a metallic coating;(b)
providing a polymeric connecting strip;(c) aligning the polymeric sheets
such that the sheet edges abut each other;(d) covering the abutting edges
with the polymeric connecting strip to form an overlapped interface,
where the polymeric connecting strip directly contacts the second side of
each polymeric sheet; and(e) applying heat and pressure to the polymeric
connecting strip to diffusion weld the polymeric sheets to the polymeric
connecting strip.

24. The method of claim 23 where the polymeric sheets and the connecting
strip comprise a thermoset polymer.

25. A method for seaming polymeric sheets comprising:(a) providing a
plurality of polymeric sheets;(b) overlapping the polymeric sheets to
produce an interface such that the polymeric sheets are in direct contact
at the interface; and(c) applying heat and pressure to the interface at
discrete spots to form a plurality of spot diffusion welds in the
interface.

26. The method of claim 25 where at least one polymeric sheet is
thermoset.

27. The method of claim 25 where at least one polymeric sheet comprises a
polyimide polymer made from pyromellitic dianhydride and oxydianiline
(PMDA/ODA).

Description:

[0001]This application is a continuation in part of, and claims priority
to, pending U.S. patent application Ser. No. 11/740,267, which was filed
on Apr. 25, 2007.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The invention relates generally to the manufacture of polymer films.
More specifically, the invention relates to polymer films that have been
seamed and a method of seaming sheets of polymeric material together with
diffusion welding.

[0004]2. Background Art

[0005]Polymer films are frequently produced in a rolled form. Such rolls
are often seamed together to combine multiple rolls, to make a longer
roll, or to make a larger polymeric sheet structure. The films can be
thin, such as 2 mils thick or less, but the films can also be thicker.
Thin sheets are often sensitive to mechanical tears and contamination.
Some materials are not as strong as others, so thin sheets are even more
prone to mechanical tears, rips, or other damages.

[0006]Thermoset plastics are polymeric materials that irreversibly cure.
The cure may result from heat, irradiation, or a reaction. The reaction
can be between two different compounds, such as with an epoxy. Some
polymers are formed with two different types of monomers, such as a
polyimide. Most polyimides are considered an AA-BB type polymer because
two different classes of monomers are used to produce the polyimide
polymer. One class of monomer is called an acid monomer, and is usually
in the form of a dianhydride. The other type of monomer is usually a
diamine, or a diamino monomer.

[0007]Thermoset plastics often have no melting point. The thermoset
plastic may decompose before melting, or the thermoset plastic may begin
decomposing as it approaches its melting point. Thermoset plastics will
generally have a glass transition temperature, where the plastic becomes
less brittle and softer above the glass transition temperature. Because
thermoset plastics do not have a true melting point, or they begin to
decompose as they approach their melting point, melt welding is not
effective in joining separate pieces. Other techniques are needed to join
sheets of thermoset plastic together. Thermoplastic materials, on the
other hand, can be melted without significant decomposition. Sheets of
thermoplastic materials can be overlapped and melted together, which is
referred to as melt welding. Thermoplastic materials do have a glass
transition temperature which is lower than its melting point. Polymeric
materials can be thermoplastic or thermoset.

[0008]Polyimides are one type of thermoset plastic with many desirable
characteristics. Other thermoset plastics include polybenzoxazoles and
epoxies. There are also examples of polyimides, polybenzoxazoles, and
epoxies which are thermoplastic. Polyimide sheets can be used for space
applications where weight is an important factor. Thinner sheets weigh
less, but still need to be strong enough to function. Also, the
temperature in space can change significantly and rapidly. If two
different materials are joined together, and the different materials have
different coefficients of thermal expansion (CTEs), the rapid and large
temperature changes can cause the materials to separate. An adhesive and
a polymer will often have different CTEs.

[0009]Thin sheets of polyimides are commonly used on satellites and other
space flight equipment. Thin sheets are also referred to as films in this
disclosure. They are typically thin polymer sheets which may have an
applied reflective metallized evaporative coating. These sheets are used
as solar shields, solar concentrators, solar sails, etc. A reflective
membrane may be used to protect orbital structures and equipment such as
satellites from direct exposure to solar radiant flux. Alternatively, a
reflective membrane may be used to concentrate solar energy on equipment
such as a solar panel that powers a satellite. Other coatings can also be
used for various applications.

[0010]Prior art techniques of seaming include bonding the materials with
an adhesive or alternatively employing a mechanical fastener. However,
the application of the adhesive seaming chemicals is challenging and
prone to contamination. Additionally, commonly used adhesives, such as
acrylic-based or silicone-based, lose mechanical strength and increase in
stiffness at temperatures below their glass transition temperature (Tg),
such as temperatures encountered in space. Differences in the CTE between
the polymer and the adhesive can also lead to premature seam failure. A
mechanical fastener, such as stitching with thread, is prone to tearing
especially if single layers of materials are seamed together.
Consequently, a method for effectively seaming thin polymer sheets is
needed.

SUMMARY OF THE INVENTION

[0011]In some aspects, the invention relates to a plurality of thermoset
plastic sheets diffusion welded together. In other aspects, the invention
relates to a method for seaming a polymeric material using diffusion
welding. In other aspects, the invention relates to a method for seaming
a polymeric material using diffusion welding, where the polymeric
material has a coating. The diffusion welding involves forming an
overlapping interface of the polymeric material, and applying heat and
pressure to the interface to form a diffusion weld between the materials.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]It should be noted that identical features in different drawings are
shown with the same reference numeral.

[0013]FIG. 1 shows a thermal seaming method that utilizes a heated stylus
in accordance with one embodiment of the present invention.

[0014]FIG. 2 shows a thermal seaming method that utilizes a heated roller
in accordance with one embodiment of the present invention.

[0015]FIG. 3 shows a thermal seaming method that utilizes a heated plunger
in accordance with one embodiment of the present invention.

[0016]FIG. 4 shows a thermal seaming method that utilizes a heated plunger
with a local tensioner.

[0017]FIG. 5 shows polymeric sheets having a core and a coating, where the
polymeric sheets are positioned to be diffusion welded.

[0018]FIG. 6 shows a composite thermoset plastic sheet with a seam.

DETAILED DESCRIPTION

The Diffusion Weld

[0019]A method for seaming thin film polymers has been developed. The
method is effective for at least some thermoset polymers which are not
conducive to melt welding. Thermoset polymers which can be seamed include
at least some polyimides and some polybenzoxazoles. Examples of thin film
polyimides include, but are not specifically limited to, PMDA/ODA
(KAPTON®), and 6FDA/4BDAF (CP1), where PMDA stands for pyromellitic
dianhydride, ODA stands for oxydianiline, 6FDA stands for
4,4'-(hexafluoroisopropylidene)diphthalic anhydride, and 4BDAF stands for
2,2-bis[4-(4aminophenoxy)phenyl] hexafluoropropane.

[0020]The method includes the application of heat and pressure to
thermally weld two layers of material together. The method produces a
weld called a "diffusion weld" that is acceptably strong and effective
under a wide range of temperature conditions. The method is effective in
seaming certain thin polymeric materials that may have a thickness of
less than 2 mils, or even thinner materials with a thickness of less than
5 microns. The method is most beneficial for thermoset polymers because
melt welding is not a viable option, but the method is also effective on
certain thermoplastic materials. The method can also be effective for
thicker polymeric materials.

[0021]"Diffusion welding" involves joining polymers by applying a
necessary amount of heat and pressure for a defined period of time. This
results in a plastic flow of material at the interface between the two
bodies of materials. The plastic flow removes, agglomerates, or buries
surface contaminants, flattens surface asperities, and brings the bodies
into intimate contact. The molecules of the separate pieces of polymeric
material are diffused and repositioned across the interface of the bodies
until the interface becomes indistinct and energy is required to separate
the bodies. Some polymeric molecules will become repositioned such that
the molecules are present in more than one polymeric piece, so the
molecules serve to bind the separate pieces together.

[0022]In order for diffusion welding to occur, the edges of the polymeric
sheet to be seamed (called a "faying surface") must be overlapped and
heated to a suitable bonding temperature. The overlapped faying surfaces
are referred to as an interface. Many polymeric materials have a distinct
glass transition temperature (Tg) where the material softens and
distorts. The suitable bonding temperature for diffusion welding can be
within about 15 degrees centigrade below the Tg to about 15 degrees
centigrade above the Tg. It is also possible to apply temperatures
more than 15 degrees centigrade above the Tg for shorter periods of
time, so the time of application and the temperature should be considered
together when forming a diffusion weld. There may also be instances where
a temperature of less than 15 degrees centigrade below the Tg can be used
with the proper application time. Once heated, pressure is added to the
interface in order to promote diffusion of the softened material across
the interface of the bodies. It is also possible to apply pressure first,
and then temperature, or to apply pressure and temperature at the same
time. The exact temperature needed, the amount of pressure needed, and
the application times for the temperature and pressure are determined by
experimentation for each different application. Factors which can affect
the temperature, pressure, and bonding time include the materials being
diffusion welded, the thickness of the materials and/or coatings, the
types of coatings, and other process conditions.

[0023]One advantage of the present invention is that a seam created by the
disclosed invention exhibits decreased tear propagation at the seam
points. Another advantage is that the present invention eliminates the
need for adhesives to join two materials. This prevents thermal
deformations due to inherent CTE differences of the materials and the
adhesive. These thermal deformations can present challenges at
temperatures below the Tg of adhesives when the modulus of the
adhesives tends to increase. Another advantage is that characteristic
conductivity of conductive materials, such as carbon-filled polyimide, is
retained at the seaming point.

Methods of Producing the Diffusion Weld

[0024]In one embodiment of the present invention, the polymeric materials
are heated by contact with a heating tool that also applies pressure to
the interface. FIG. 1 shows one example of a thermal seaming system 10
that uses a heated stylus 16. A first sheet of a polymeric material 12a
and a second sheet of polymeric material 12b are positioned with their
respective faying surfaces forming an overlapping interface 14. The
overlapped interface 14 can be about one inch across, but wider overlaps
allow more area for a larger diffusion weld, and smaller overlaps can
also be used. The interface 14 is placed on an insulative backing 18 and
a protective overlay 20 can be placed over the top of the interface 14.
The heated stylus 16 is then dragged 24 across the overlay 20. The stylus
16 applies both heat and pressure to the interface 14 and forms a
continuous diffusion weld 22.

[0025]FIG. 2 shows an alternative thermal seaming system embodiment 30 of
the present invention that utilizes a heated wheel 32. In this
embodiment, a heated wheel 32 is rolled 34 across the interface 14 to
apply both heat and pressure that forms a continuous diffusion weld 22.
While this embodiment is shown without a protective overlay, it should be
understood that embodiments exist for diffusion welding with or without a
protective overlay. The need for a protective overlay is determined by
experimentation based on several factors, including the materials being
diffusion welded, the thickness of the materials, the type of apparatus
applying the heat and pressure, the diffusion welding temperature and
pressure used, and the application time for the temperature and pressure
application.

[0026]FIG. 3 shows yet another thermal seaming system embodiment 40 of the
present invention that utilizes a plurality of heated plungers 42.
Alternate embodiments could use a single heated plunger 42. In this
embodiment, the heated plungers 42 move intermittently in vertical 44 and
horizontal 46 directions applying heat and pressure at various points
along the interface 14. The effect is to create multiple "spot" diffusion
welds 48. The plungers 42 apply heat and pressure to produce spot
diffusion welds 48, and then move vertically 44 out of contact with the
interface 14. The plungers 42 then move horizontally 46 to re-position
themselves for the next spot diffusion welds 48. Then the plungers 42
move vertically 44 downward into contact with the interface 14, and
produce the next spot diffusion welds 48 by applying heat and pressure.
Reference to the plunger 42 moving horizontally or vertically means the
plunger motion includes a horizontal or vertical component, so the
plunger 42 can move both horizontally and vertically at the same time.
Vertical motion is used to contact or separate the plunger 42 from the
interface, and horizontal motion is used to re-position the plunger 42
for another spot diffusion weld 48.

[0027]The plungers 42 can be used to produce spot diffusion welds 48,
instead of the continuous diffusion welds discussed above. The plungers
42 could also be dragged to produce continuous diffusion welds, or a
mixture of spot and continuous diffusion welds, as desired. The plungers
42 can provide simpler temperature control mechanisms, because the
diffusion welding process is intermittent and focused at the end of the
plunger 42. Also, the contact end of the plunger 42 can have a small
area, so a high pressure can be applied at the spot of the diffusion
weld. However, the spot diffusion welds 48 may not produce a complete
seal between the first and second polymeric sheets 12a, 12b. Each spot
diffusion weld 48 may be isolated from every other spot diffusion weld
48, meaning each spot diffusion weld 48 does not contact any other spot
diffusion weld 48. When the spot diffusion welds 48 are isolated, a
complete seal may not be formed between the first and second polymeric
sheets 12a, 12b,

[0028]Most welds of materials with a non-zero CTE have differential
shrinkage distortion of the joined bodies upon cooling. This is the
result of temperature gradients which are characteristic in practically
all welding techniques. This distortion is undesirable for many
applications using diffusion welded thin film membranes. It can be
reduced by locally tensioning the sheets of polymeric material 12a, 12b
in the vicinity of the diffusion weld during the welding process. When
the tension is released, the local residual compression stress tends to
counteract the local residual tensile stresses that result from the weld.
FIG. 4 shows one embodiment of the thermal seaming system 50 that
utilizes a heated plunger 42 in conjunction with a local tensioner 52. In
this embodiment, the tensioner 52 is circular-shaped. The plunger 42
contacts the interface 14 through a tensioner opening 54 to create the
diffusion weld 48.

[0029]The examples shown in FIGS. 1-4 demonstrate diffusion welding of
shear or overlapping joints. Other embodiments of the present invention
could be used to produce butt or tensile joints. In these embodiments,
the faying surfaces are abutted rather than overlapped, so the polymeric
sheets are positioned edge to edge. A separate seaming sheet can then be
positioned over the abutted edges of the polymeric sheets, so there is an
overlapped interface. The heated bonding tool is brought in contact with
the overlapped interface to form a diffusion weld.

[0030]While these embodiments show a single device that provides both heat
and pressure to the interface, it should be understood that alternative
embodiments could use multiple devices to provide these as desired.
Further, the temperatures and pressures applied will depend on the
specific characteristics of each material. However, it is common for the
suitable bonding temperature to exceed 500 F yet still not reach the
melting point of the material. If two dissimilar materials, such as a
polyimide and non-polyimide, are being bonded then the material with the
lower glass transition temperature (Tg) and/or melting point should
be heated to a lesser temperature. One way this may be accomplished is by
placing a heat source directly on the material with the higher Tg
while placing the lower Tg material underneath the higher Tg
material to avoid direct contact with the heat source. The material with
the higher Tg serves to insulate the material with the lower Tg
to some extent.

Polymeric Materials

[0031]While the present invention has been described for diffusion welding
two or more similar thermoset plastic materials, the methods disclosed
may also be effective to diffusion weld two or more thermoplastic plastic
materials or even to diffusion weld two or more dissimilar materials. For
example, the various embodiments could be used to connect a polyimide
with a different polyimide material such as diffusion welding the
polyimide associated with the trademark KAPTON® and CP1 together.
Other embodiments may involve seaming a thermoset plastic to any of the
following polymers: polyamides (such as polymer associated with the trade
name NYLON); polyesters such as poly(ethylene naphthalate) (PEN) or
poly(ethylene terephthalate) (PET/polymers known by the trade name
MYLAR); polyamide imide (PAI/polymers known by the trade name TORLON);
polyether ketone (PEK); polyether ketone ketone (PEKK); polyether ether
ketone (PEEK); polyether imide (PEI); polyphenylene sulfide (PPS);
polyether sulfone (PES); polytetrafluoroethylene (PTFE); and
polyphenylene (PARMAX). These materials are given as examples of
alternative materials that may be diffusion welded to a thermoset plastic
or to a thermoplastic plastic. It should be understood that other
materials known in the art with similar performance characteristics could
be used as well.

[0032]It is possible to diffusion weld thermoset materials with
thermoplastic materials if the characteristics of the different polymers
are acceptable. For example, some polyimides associated with the
trademark KAPTON® have a Tg higher than 300 degrees Celsius, and
these KAPTON® polyimides can be diffusion welded to other materials
with a melting point lower than the Tg of KAPTON®. It is also
possible to diffusion weld certain thermoplastic materials together. This
can include separate sheets of one type of thermoplastic polymer, or
separate sheets of different types of thermoplastic polymers.

[0033]Many polymeric sheets will include coatings of one type or another,
and these coatings can affect diffusion welds. A polymeric sheet with a
coating includes at least a core 60 and a coating 62, as seen in FIG. 5,
where the core 60 is the base layer and the coating 62 is applied to the
core 60. Coatings 62 can be on one side of a polymeric sheet 12a, or they
can be on both sides of a polymeric sheet 12a. There can also be multiple
coatings 62 on a polymeric sheet 12a. A polymeric sheet 12a can have a
first type of coating 62 on a first side 64, and a different type of
coating on the second side 66. Each side 64, 66 can have zero, one, two,
or more coatings. The coatings can change the properties of the polymeric
sheet 12a, so different coatings 62 and cores 60 are used for different
applications.

[0034]Some coatings 62 can be at the interface 14 and not interfere with
the diffusion weld, but other coatings can prevent or weaken the
diffusion weld if present at the interface 14. For example, metallic
coatings should not be between the layers in the interface 14 where the
diffusion weld is produced. The metallic coating can be on a polymeric
sheet 12a on a side facing away from another layer, so the metallic
coating is not between layers in the interface 14. The coating 62 on
polymeric sheet 12a is facing away from polymeric sheet 12b, and so the
coating 62 on polymeric sheet 12a is not between layers in the interface
14. However, polyimide cores 60 with a polytetrafluoroethylene (PTFE)
coating 62 or another fluorinated polymer, such as fluorinated ethylene
propylene (FEP) and perfluoroalkoxy (PFA) can be diffusion welded with
the coating 62 between the layers in the interface 14.

[0035]Polymeric sheets with metallic coatings cannot be diffusion welded
if the metallic coating is between the polymers of the different sheets.
One way to diffusion weld polymeric sheets with metallic coatings is to
remove the metallic coating at the faying surface, so there is no coating
in the overlapped interface. Alternatively, the polymeric sheets can be
positioned with the faying surfaces abutted instead of overlapped, so the
sheets are edge to edge. A separate seaming sheet can be positioned over
the abutting edges on the side of the polymeric sheets which do not have
a metallic coating, and the diffusion weld can be made in the interface.
Other alternatives include coating the polymeric sheets after they have
been seamed, or preventing a faying surface from being coated when the
rest of the polymeric sheet receives the metallic coating.

[0036]Most experiments completed to date have involved diffusion welding
two separate sheets of the same polymeric material. Diffusion welds have
been found effective for certain materials, but more experimentation is
needed to determine what materials and what mix of materials can be
diffusion welded. Below is a list of materials for which the diffusion
welding described above has been shown to work.

[0047]Thermoset plastics seamed together with diffusion welds can be
produced with the techniques described above, as shown in FIG. 6. This
would include a plurality of component thermoset plastic sheets, such as
a first component thermoset plastic sheet 70 and a second component
thermoset plastic sheet 72. A plurality of component thermoset plastic
sheets 70, 72 can be diffusion welded together to form one larger
composite thermoset plastic sheet 74. The composite thermoplastic sheet
74 can be recognized because there is no adhesive between the overlapping
layers in the interface 14. The thermoset plastic of one component sheet
70 is directly bonded to the thermoset plastic of another component sheet
72. If melt welding had been attempted, weld points would show increased
levels of decomposition. The use of stitching or other mechanical
connectors is not needed, but could be used with diffusion welding if
desired.

[0048]The composite thermoset plastic sheet 74 includes a seam 76 with
overlapping layers of thermoset plastic. The seam 76 may not include any
adhesive, but adhesive could also be added if desired. A seam 76 differs
from an interface 14 in that a seam 76 includes a connection between at
least two separate sheets, and an interface 14 is an area with
overlapping layers either before or after the layers are connected. At
least one of the component thermoset plastic sheets, such as the first
component thermoset plastic sheet 70, may comprise a polyimide, but the
thermoset plastic may also comprise polybenzoxazoles, polyimide filled
polytetrafluoroethylene, other thermoset polymers, or combinations of the
above.

[0049]A component thermoset plastic sheet 70 and/or 72 may also comprise a
coating 62. Reference is now made to the first component thermoset
plastic sheet 70 having the coating 62, but it is to be understood the
second component thermoset plastic sheet 72 may have a coating 62 either
instead of the first component thermoset plastic sheet 70, or as well as
the first component thermoset plastic sheet 70. The coating 62 on the
first component thermoset plastic sheet 70 can be in direct contact with
the second component thermoset plastic sheet 72 at the seam 76. The
coating may comprise polytetrafluoroethylene, polyimides,
polybenzoxazoles, or other polymers. Other coatings 62, such as metallic
coatings or certain other inorganic coatings, may be on the composite
thermoset plastic sheet 74 but not present between the layers of the seam
76. The coating 62 shown on the second component thermoset plastic sheet
72 is not present between the layers of the seam 76. As such, a coating
62 not present between the layers of the seam 76 would not directly
contact another component thermoset plastic sheet 74.

[0050]The composite thermoset plastic sheet 74 may be no more than 2 mils
thick, and may even be less than 5 microns thick. It is also possible for
the composite thermoset plastic sheet 74 to have a thickness greater than
2 mils. Reference to the thickness of the composite thermoset plastic
sheet thickness refers to the thickness in the body of the composite
thermoset plastic sheet 74, and not the thickness at the seam 76.

[0051]In one embodiment, the seam 76 is made from overlapping different
edges of one component thermoset plastic sheet, such that the composite
thermoset plastic sheet 74 forms a loop or belt. In this embodiment, the
first component thermoset plastic sheet 70 and the second component
thermoset plastic sheet 72 are represented by different edges of one
single component thermoset plastic sheet. Alternatively, more than one
component thermoset plastic sheets can be connected to form a loop or
belt. Many other shapes can also be made by connecting different edges of
component thermoset plastic sheets, such as a cube or a pyramid.

[0052]A composite plastic sheet can be formed between a component
thermoset plastic sheet and a component thermoplastic plastic sheet. The
composite plastic sheet therefore could include a component thermoset
plastic sheet and a component thermoplastic plastic sheet diffusion
welded together at the seam. Coatings can be applied similar to the
component thermoset plastic sheets diffusion welded together to form a
composite thermoset plastic sheet 74.

WORKING EXAMPLES

[0053]Specific working examples are provided below to further clarify
specific embodiments of the current invention.

Example 1

[0054]Spot diffusion welds were produced in an overlapping seam between a
2 mil thick polyimide film associated with the trademark KAPTON HN and
another 2 mil thick polyimide film associated with the trademark KAPTON
HN. The overlapping seam was held in place by magnets placed above and
below the seam such that the magnets were drawn towards each other. A
backing was positioned under the seam such that the plunger presses the
seam directly into the backing. The backing was formed of a 1/1000 inch
thick KAPTON tape secured with a silicon pressure sensitive adhesive to a
1/16 inch thick ceramic wick. The 1/16 inch ceramic wick was positioned
over a 1/8 inch thick nylon sheet, which was positioned over a rubber
foam.

[0055]A plurality of heated plungers was pressed into the polyimide
material at the seam, such that the polyimide seam was positioned
directly between the heated plungers and the backing. The heated plungers
had a tip with a 0.145 inch diameter circle, and the tip was heated to
650 degrees centigrade. The heated tip of the plunger was applied to the
seam with a weight of 390 grams per tip for a total application time of 5
seconds. The heated plungers were then retracted from the seam, and
repositioned for another spot weld. The process was repeated until the
two polyimide sheets were thermally welded together.

Example 2

[0056]Spot diffusion welds were produced in an overlapping seam between a
2 mil thick polyimide film associated with the trademark KAPTON E and
another 2 mil thick polyimide film associated with the trademark KAPTON
E. The same process and parameters as described for Example 1 was used in
Example 2.

Example 3

[0057]Spot diffusion welds were produced in an overlapping seam between a
2 mil thick polyimide film associated with the trademark KAPTON HN and a
2 mil thick polyimide film associated with the trademark KAPTON E. The
same process and parameters as described for Example 1 was used in
Example 3.

Example 4

[0058]Spot diffusion welds were produced in an overlapping seam between a
2 mil thick polyimide film associated with the trademark KAPTON HN and a
1 mil thick polyimide film associated with the trademark KAPTON HN. The
same process and described for Example 1 was used in Example 4, except
the plunger tip temperature was 630 degrees centigrade, the weight per
tip was 300 grams, and the application time was 4 seconds.

[0059]While the invention has been described with respect to a limited
number of embodiments, those skilled in the art, having benefit of this
disclosure, will appreciate that other embodiments can be devised which
do not depart from the scope of the invention as disclosed here.
Accordingly, the scope of the invention should be limited only by the
attached claims.